The general idea of electrically isolating a patch of membrane and studying the ion channels in that patch under voltage-clamp conditions was outlined by Neher, Sakmann, and Steinbach in “The Extracellular Patch Clamp, A Method For Resolving Currents Through Individual Open Channels In Biological Membranes”, Pflueger Arch. 375; 219-278, 1978. The authors found that, by pressing a pipette containing acetylcholine (ACh) against the surface of a muscle cell membrane, they could see discrete jumps in electrical current that were attributable to the opening and closing of ACh-activated ion channels. However, they were limited in their work by the fact that the resistance of the seal between the glass of the pipette and the membrane (10-50 MΩ) was very small relative to the resistance of the channel (10 GΩ). The electrical noise resulting from such a seal is inversely related to the resistance and was large enough to obscure the currents flowing through ion channels, the conductance of which is smaller than that of the ACh channel. It also prohibited the clamping of the voltage in the pipette to values different from that of the bath due to the large currents through the seal that would result.
It was then discovered that by fire polishing the glass pipettes and by applying suction to the interior of the pipette a seal of very high resistance (1-100 GΩ) could be obtained with the surface of the cell. This giga-seal reduced the noise by an order of magnitude to levels at which most channels of biological interest can be studied and greatly extended the voltage range over which these studies could be made. This improved seal has been termed a “giga-seal”, and the pipette has been termed a “patch pipette”. A more detailed description of the giga-seal may be found in O. P. Hamill, A. Marty, E. Neher, B. Sakmann & F. J. Sigworth: Improved patch-clamp techniques for high resolution current recordings from cells and cell-free membrane patches. Pflugers Arch. 391, 85-100, 1981. For their work in developing the patch clamp technique, Neher and Sakmann were awarded the 1991 Nobel Prize in Physiology and Medicine.
Ion channels are transmembrane proteins which catalyse transport of inorganic ions across cell membranes. The ion channels participate in processes as diverse as generating and timing action potentials, synaptic transmission, secretion of hormones, and contraction of muscles. Many drugs exert their specific effects via modulation of ion channels. Examples are antiepileptic compounds like phenytoin and lamotrigine which block voltage-dependent Na+-channels in the brain, antihypertensive drugs like nifedipine and diltiazem which block voltage dependent Ca2+-channels in smooth muscle cells, and stimulators of insulin release like glibenclamide and tolbutamide which block an ATP-regulated K+-channel in the pancreas. In addition to chemically induced modulation of ion-channel activity, the patch clamp technique has enabled scientists to perform manipulations with voltage dependent channels. These techniques include adjusting the polarity of the electrode in the patch pipette and altering the saline composition to moderate the free ion levels in the bath solution.
The patch clamp technique represents a major development in biology and medicine, since this technique allows measurement of ion flow through single ion channel proteins, and also allows the study of the single ion channel responses to drugs. Briefly, in standard patch clamp technique, a thin (approx. 0.5-2μ in diameter) glass pipette is used. The tip of this patch pipette is pressed against the surface of the cell membrane. The pipette tip seals tightly to the cell and isolates a few ion channel proteins in a tiny patch of membrane. The activity of these channels can be measured individually (single channel recording) or, alternatively, the patch can be ruptured, allowing measurements of the channel activity of the entire cell membrane (whole-cell configuration). High-conductance access to the cell interior for performing whole-cell measurements can be obtained by rupturing the membrane by applying negative pressure in the pipette.
During both single channel recording and whole-cell recording, the activity of individual channel subtypes can be characterised by imposing a “voltage clamp” across the membrane. In the voltage clamp technique the membrane current is recorded at a constant membrane potential. Or—to be more precise—the amplifier supplies exactly the current which is necessary to keep the membrane potential at a level determined by the experimenter. Hence, currents resulting from opening and closing of ion channels are not allowed to recharge the membrane.
A major limitation determining the throughput of the patch clamp technique is localisation and clamping of cells and pipette, and the nature of the fluidics system, which leads the dissolved compound to cells and patches. In conventional patch clamp setups, cells are placed in experimental chambers, which may be continuously perfused with a physiological salt solution. The establishment of the cell-pipette connection in these chambers is time consuming and troublesome. Compounds are applied by changing the inlet to a valve connected to a small number of feeding bottles. The required volumes of the solution and the amount of compound to be tested are high (i.e., several hundreds of milliliters).
High throughput systems for performing patch clamp measurements have been proposed, which typically consist of a substrate with a plurality of sites adapted to hold cells in a measuring configuration where the electrical properties of the cell membrane can be determined.
U.S. Pat. No. 5,187,096, Rensselaer, discloses an apparatus for monitoring cell-substrate impedance of cells. Cells are cultured directly on the electrodes, which are then covered with a plurality of cells; thus, measurements on individual cells cannot be performed.
WO 98/54294 discloses a substrate with wells containing electrode arrays. The substrate with wells and electrodes are formed in silicon using CVD (Chemical Vapor Deposition) and etching techniques and comprises Silicon Nitride “passivation” layers surrounding the electrodes. Cells are cultivated directly on the electrode array. The substrate is adapted to measure electrophysiological properties and discloses a variety of proposed measuring schemes.
WO 99/66329, Cenes, discloses a substrate with perforations arranged in wells and electrodes provided on each side of the substrate. The substrate is formed by perforating a silicon substrate with a laser and may be coated with anti-adhesive material on the surface. The substrate is adapted to establish giga-seals with the cells by positioning the cells on the perforations using suction creating a liquid flow through the perforations, providing the anti-adhesion layer surrounding the perforations, or by guiding the cells electrically. The cells can be permeabilised by EM fields or chemical methods in order to provide a whole-cell-measuring configuration. All perforations, and hence all measurable cells, in a well share one working electrode and one reference electrode.
WO 99/31503, Vogel et al., discloses a measuring device with a passage arranged in a well on a substrate (carrier) and separating two compartments. The measuring device comprises two electrodes positioned on either side of the passage and adapted to position a cell at the passage opening. The substrate may have hydrophobic and hydrophilic regions in order to guide the positioning of the cells at the passage opening.
U.S. Pat. No. 6,932,893 (US'893) relates to a high throughput system for determining and/or monitoring electrophysiological properties of ion channels of ion channel-containing membranes, typically lipid membrane-containing structures such as cells. The system provides means for performing an automated process including preparation of the cells, preparation of the measuring configuration, and performing the measurements on a large number of cells independently. Also, the US'893 invention relates to a substrate and a method for establishing an electrophysiological measuring configuration in which a cell membrane forms a high resistive seal in a configuration with measuring electrodes, making it possible to determine and monitor a current flow through the cell membrane. More particularly, the invention relates to such a substrate, which provides means for automatically positioning cells at measuring sites using electroosmotic flow. Also, the invention relates to a main electric circuit for performing testing and measurements on cells at a plurality of sites in parallel.